This invention relates to an improvement in a field emission electron gun to be used for a scanning type electron microscope and the like.
The field emission electron gun has the excellent feature that in comparison with a thermal electron gun, it has a brightness 10.sup.3 times as high as that of the latter. To put the field emission electron gun to practical use, however, it is necessary to place the gun under a high vacuum of below 10.sup.-9 Torr. To maintain this high vacuum during emission of electrons, furthermore, it is an essential requirement to prevent gases emitted from the anode due to electron bombardment from contaminating the atmosphere of the electron gun in the vicinity of the cathode.
As a method of preventing such gas emission, there has conventionally been known a heating degassing method in which the anode is heated by heating means, such as a heater, so as to perform degassing. According to this method, however, large quantities of gases are emitted from the heating means itself during the treatment of the anode, thereby lowering the vacuum and thus failing to maintain the high vacuum state.